This invention relates generally to a method of improving response in chromatographic analysis and more particularly to a sample modulator cell for synchronous detection with improved signal-to-noise ratio.
Two problems that limit the utility of detectors in gas chromatography are drift and noise. Examples of detectors that are subject to drift are the thermal conductivity detector (TDC) and the thermionic specific detector (TSD). The drift in the TCD is caused by small changes in the temperature of the cell wall. The drift in the TSD is caused by small changes occurring on the surface of the bead. These changes are both thermal and chemical. Examples of detectors in which the dominant noise source is within the detector cell are the flame photometric detector (FPD), flame ionization detector (FID) and the TSD. The noise in the FPD is caused by random low frequency fluctuations in the luminosity of the background flame. The TSD likewise suffers from fluctuations in the nature of the bead surface.
When the drift and noise sources are inherent to the detector and not to the electronics which controls the detector or amplifies the signal therefrom, it has been known to minimize or eliminate these problems by AC techniques. U.S. Pat. No. 3,740,154 issued to J. A. Green, for example, discloses a flame photometer whereby the gas to be analyzed is modulated in a modulator equipped with a flexible sinusoidally oscillating membrane. Since the sample gas comes into direct contact with this membrane, this modulator cannot be used for certain samples, such as caustic ones and those at a very high temperature. Fluid control systems not requiring a flexible membrane of the aforementioned type have been disclosed, for example, in U.S. Pat. No. 3,357,233 issued to L. B. Roof, and U.S. Pat. No. 4,309,898 issued to R. L. Horton, but both these devices are fluidic in nature and require a stream which is greater by orders of magnitude in order to properly operate.
U.S. Pats. Nos. 4,316,381 and 4,316,382 issued to T. A. Woodruff disclose a modulated detector which is provided with a storage volume for modulation in the fluid path so that the measurements are made only at times in which the flow has stopped. This is a static measurement, not a continuous dynamic process which allow shorter response times.